SEOUL (Reuters) - The first distress call from a sinking South Korean ferry was made by a boy with a shaking voice, three minutes after the vessel made its fateful last turn.

He called the emergency 119 number which put him through to the fire service, which in turn forwarded him to the coastguard two minutes later. That was followed by about 20 other calls from children on board the ship to the emergency number, a fire service officer told Reuters.

The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the traditional honeymoon island of Jeju.

The boy who made the first call, with the family name of Choi, is among the missing. His voice was shaking and sounded urgent, a fire officer told MBC TV. It took a while to identify the ship as the Sewol.

"Save us! We're on a ship and I think it's sinking," Yonhap news agency quoted him as saying.

The fire service official asked him to switch the phone to the captain, and the boy replied: "Do you mean teacher?"

The pronunciation of the words for "captain" and "teacher" is similar in Korean.

The captain of the ship, Lee Joon-seok, 69, and other crew members have been arrested on negligence charges. Lee was also charged with undertaking an "excessive change of course without slowing down".

Authorities are also investigating the Yoo family, which controls the company that owns the ferry, Chonghaejin Marine Co Ltd, for possible financial wrongdoing amid growing public scrutiny.

An official at the Financial Supervisory Service (FSS) told Reuters it was investigating whether Chonghaejin or the Yoo family engaged in any illegal foreign exchange transactions. The official did not elaborate.

Another person familiar with the matter told Reuters that prosecutors were looking into suspected tax evasion by the firm, its affiliates or the Yoo family with assistance from the National Tax Service. A spokesman at the tax agency declined to comment on the matter.

"There are lots of reports in the media, so as the regulator we need to check if they are true," another FSS official said.

Neither the Yoo family nor the company was immediately available for comment.

ONLY OBEYING ORDERS

Several crew members, including the captain, left the ferry as it was sinking, witnesses have said, after passengers were told to stay in their cabins. President Park Geun-hye said on Monday that instruction was tantamount to an "act of murder".

Many of the children did not question their elders, as is customary in hierarchical Korean society. They paid for their obedience with their lives.

Four crew members appeared in court on Tuesday and were briefly questioned by reporters before being taken back into custody. One unidentified second mate said they had tried to reach the lifeboats, but were unable to because of the tilt.

Only two of the vessel's 46 lifeboats were deployed.

Two first mates, one second mate and the chief engineer stood with their heads lowered and it was impossible to tell who was speaking.

One said there had been a mistake as the boat made a turn. Another said there was an eventual order to abandon ship. He said the crew gathered on the bridge and tried to restore balance, but could not.

"Maybe the steering gear was broken," one said.

Media said the ship lost power for 36 seconds, which could have been a factor.

Public broadcaster KBS, quoting transcripts of the conversation between the crew and sea traffic control, the Jindo Vessel Traffic Services Centre, said the passengers were told repeatedly to stay put.

For half an hour, the crew on the third deck kept asking the bridge by walkie-talkie whether or not they should make the order to abandon ship, KBS said.

No one answered.

"We kept trying to find out but … since there was no instruction coming from the bridge, the crew on the third floor followed the instructions on the manual and kept making 'stay where you are' announcements," KBS quoted a crew member as saying. "At least three times."

Lee was not on the bridge when the ship turned. Navigation was in the hands of a 26-year old third mate who was in charge for the first time on that part of the journey, according to crew members.

In a confused exchange between the sinking Sewol and maritime traffic control released by the government, the crew said the ship was listing to port.

"Make passengers wear life jackets and get ready in case you need to abandon ship," traffic control said.